The best advice I can offer is to be polite and courteous when you approach farmers or landowners.
If possible, write or telephone beforehand to make an appointment and, when you visit, dress smartly and look respectable.
Don’t turn up in camouflage, with your gun and a bag of decoys in the car.
Be prepared to offer assurance that you are a responsible shooter; this may include giving the name of another farmer as a reference, or demonstrating that you are a member of a local shooting or wildfowling club.
It will certainly include carrying a membership card to show that you belong to an organisation such as BASC or the CA, and that you therefore have public liability insurance.
Farmers who grow oilseed rape or peas may well be happy for you to shoot pigeon or rooks on their land.
See which crops they are growing and watch to see where birds are feeding.
If you are prepared to shoot other than at weekends, you stand a good chance.
Finally, respect the farmer’s wishes.
Do not shoot any quarry except that for which you have permission.